During the past decade, cosmology has unquestionably entered the domain of high-precision science. Just a few years ago several basic cosmological quantities, such as the expansion parameter, H, and the flatness parameter, , were known only to within a factor of 2. Now new observations using WMAP, SDSS, and the high-redshift type Ia supernovae measure these and other crucial quantities with percent-level accuracy. Several of inflation's most basic quantitative predictions, including = 1 and ns 1, may now be compared with data that are discriminating enough to distinguish inflation from many of its theoretical rivals. So far, every measure has been favorable to inflation.
Even with the evidence in favor of inflation now stronger than ever, much work remains. Inflationary cosmology has always been a framework for studying the interconnections between particle physics and gravitation - a collection of models rather than a unique theory. The next generation of astronomical detectors should be able to distinguish between competing inflationary models, whittling down the large number of options to a preferred few. One important goal is the high-precision measurement of polarization effects in the CMB, which allows the possibility of uncovering the traces of gravity waves originating from inflation. Gravity waves of the right pattern would be a striking test of inflation, and would allow us to determine the energy density of the "false vacuum" state that drove inflation. The new cosmological observations also offer physicists one of the best resources for evaluating the latest developments in idea-rich (but data-poor) particle theory, where much of the current research has been aimed at the high-energy frontier, well beyond the range of existing accelerators. Perhaps the interface between string theory and cosmology will lead to new predictions for the astronomers to test. Whether such tests are successful or not, physicists are certain to learn important lessons about the nature of space, time, and matter.
Meanwhile, several major puzzles persist. Now that physicists and astronomers are confident that = 1 to high accuracy, the question remains of just what type of matter and energy is filling the universe. Ordinary matter, such as the protons, neutrons, and electrons that make up atoms, contributes just 4% to this cosmic balance. Nearly one-quarter of the universe's mass-energy is some form of "dark matter" - different in kind from the garden-variety matter we see around us, and yet exerting a measurable gravitational tug that shapes the way galaxies behave. Particle physicists have offered many candidates for this exotic dark matter, but to date no single contender has proved fully convincing .
Even more bizarre is the dark energy now known to contribute about 70% to . This dark energy is driving a mini-inflationary epoch today, billions of years after the initial round of inflation. Today's accelerated expansion is far less fast than the earlier inflationary rate had been, but the question remains why it is happening at all. Could the dark energy be an example of Einstein's cosmological constant? Or maybe it is a variation on an inflationary theme: Perhaps some scalar field has been sliding down its potential energy hill on a time scale of billions of years rather than fractions of a second . Whatever its origin, dark energy, much like dark matter, presents a fascinating puzzle that will keep cosmologists busy for years to come.
The authors thank Gia Dvali, Hong Liu, Max Tegmark, Sandip Trivedi, and Henry Tye for very helpful comments on the manuscript. This work was supported in part by funds provided by the U.S. Department of Energy (D.O.E.) under cooperative research agreement #DF-FC02-94ER40818.